fullscreen: remove Linux menubar workaround

It appears this is causing issues 1) on recent Ubuntu 22.04 and also 20.04 with the defaut window
manager, as well as 2) with (unofficial) window manager extensions that enable the so-called 'global menu'
(menubar is integrated in the desktop's top bar) after toggling fullscreen, e.g. when switching skins
while in fullscreen mode:
* 1) 2) menubar hotkeys not working anymore
* 2) menubar is not restored in the title bar but in the windwo itself

src/mixxx.cpp

@@ -168,6 +168,9 @@ MixxxMainWindow::MixxxMainWindow(QApplication* pApp, const CmdlineArgs& args)
           m_toolTipsCfg(mixxx::TooltipsPreference::TOOLTIPS_ON),
           m_runtime_timer("MixxxMainWindow::runtime"),
           m_cmdLineArgs(args),
+#ifdef __LINUX__
+          m_recreateMenubarOnFullscreenToggle(false),
+#endif
           m_pTouchShift(nullptr) {
     m_runtime_timer.start();
     mixxx::Time::start();
@@ -213,6 +216,18 @@ MixxxMainWindow::MixxxMainWindow(QApplication* pApp, const CmdlineArgs& args)
     initializeKeyboard();
     installEventFilter(m_pKeyboard);
 
+#ifdef __LINUX__
+    // If true this will recreate and reconnect the mainmenu bar when toggling
+    // fullscreen in order to fix
+    // "No menu bar with ubuntu unity in full screen mode"
+    // https://github.com/mixxxdj/mixxx/issues/6072 (bugs #885890 and #1076789).
+    // Before touching anything here, please read those bugs.
+    // 'unity' seems to be the only affectde desktop environment
+    // Alternative global menus work without this currently, e.g. vala-panel-appmenu
+    m_recreateMenubarOnFullscreenToggle =
+            QString::fromLocal8Bit(qgetenv("XDG_SESSION_DESKTOP")) == "unity";
+#endif
+
     // Menubar depends on translations.
     mixxx::Translations::initializeTranslations(
         m_pSettingsManager->settings(), pApp, args.getLocale());
@@ -1426,23 +1441,25 @@ void MixxxMainWindow::slotViewFullScreen(bool toggle) {
     if (isFullScreen() == toggle) {
         return;
     }
+    qInfo() << "     ~" << m_recreateMenubarOnFullscreenToggle;
 
     if (toggle) {
         showFullScreen();
 #ifdef __LINUX__
-        // Fix for "No menu bar with ubuntu unity in full screen mode" Bug
-        // #885890 and Bug #1076789. Before touching anything here, please read
-        // those bugs.
-        createMenuBar();
-        connectMenuBar();
-        if (m_pMenuBar->isNativeMenuBar()) {
-            m_pMenuBar->setNativeMenuBar(false);
+        if (m_recreateMenubarOnFullscreenToggle) {
+            createMenuBar();
+            connectMenuBar();
+            if (m_pMenuBar->isNativeMenuBar()) {
+                m_pMenuBar->setNativeMenuBar(false);
+            }
         }
 #endif
     } else {
 #ifdef __LINUX__
-        createMenuBar();
-        connectMenuBar();
+        if (m_recreateMenubarOnFullscreenToggle) {
+            createMenuBar();
+            connectMenuBar();
+        }
 #endif
         showNormal();
     }

src/mixxx.h

@@ -181,6 +181,10 @@ class MixxxMainWindow : public QMainWindow {
 
     const CmdlineArgs& m_cmdLineArgs;
 
+#ifdef __LINUX__
+    bool m_recreateMenubarOnFullscreenToggle;
+#endif
+
     ControlPushButton* m_pTouchShift;
     mixxx::ScreenSaverPreference m_inhibitScreensaver;